Motion sickness is common is among users of simulators and virtual environments. Many simulators and virtual environments are characterized by the presentation of information through visual displays, so that motion sickness in these systems falls within the larger category of visually-induced motion sickness. Visually-induced motion sickness causes health problems for users of high-technology systems (including applications in medicine, flight training, and entertainment). These problems can lead to health risks for individuals (e.g., due to disorientation and nausea, users of military flight simulators are sometimes forbidden to operate aircraft or automobiles for several hours following simulator use). Additional problems include delayed or limited acceptance of new simulation and virtual environment technologies. The long-term objectives of the proposed research include a theoretically-based understanding of the causes of visually-induced motion sickness, with the ultimate goal being the development of objective measurement techniques that will permit reliable, accurate, inexpensive, non-invasive identification of individuals that are at risk for visually-induced motion sickness. The specific objectives include understanding of the conditions under which instabilities in the control of standing posture precede the subjective symptoms of visually-induced motion sickness. Experiments will investigate relations between postural instability and visually-induced motion sickness, focusing on the role of measurable instabilities as predictors of susceptibility to motion sickness. Subjects' postural motion will be measured prior to and during exposure to nauseogenic visual stimuli. The independent variables will be the amplitude and frequency of imposed visual motion, and whether or not subjects are passively restrained. The experimental' hypotheses focus on the combinations of frequency and amplitude that may produce postural instability and motion sickness, and on particular types of instability that may be expected.